Pulmonary artery hypertension in children or adults (PAH) is a progressive and fatal disease characterized by sustained elevations of pulmonary artery pressure of unknown etiology. Vasodilators are the mainstay of therapy. However 20-30% of patients do not respond to vasodilators and non-responders have a poor prognosis eventually requiring lung transplantation. As the pathobiology is unknown, vasodilator therapy has significant morbidity and cost and specific diagnostic/prognostic biomarkers are lacking, our long-term goal is to identify circulating, diagnostic and prognostic markers of IPAH to improve long term outcomes. To pursue this goal, we have optimized a non-biased, in-depth proteomics approach in patients with pulmonary hypertension. In our preliminary data using longitudinal plasma samples of children with IPAH we have already identified significant changes in a number of plasma proteins in patients with IPAH. Therefore, we hypothesize that plasma samples from patients with IPAH will contain diagnostic/prognostic lung and endothelial cell specific biomarkers. To examine this hypothesis we will use a non-biased proteomics approach to identify unique circulating PAH biomarker proteins from the lung and specifically pulmonary arterial endothelial cells (PAEC). In Aim 1. Using a state of the art quantitative biomarker development pipeline, we will identify circulating lung and in particular, PAEC specific biomarkers of IPAH by comparing proteomes of matched plasma, lung and PAEC harvested from PHBI Repository patients transplanted for IPAH (n=10) or APAH (n=10) related to congenital heart disease and (n=10) and normal non-transplanted donors (n=10) as controls. In Aim 2. we will verify candidate plasma IPAH biomarkers using high throughput quantitative multiplex MS-based (multple reaction montioring (MRM) assays using the remaining PHBI Repository of plasma samples to verify the protein identification, it's efficacy in both children and adults and determine it's specificity for IPAH vs non-IPAH. The innovation is that this proposal applies a state of the art unbiased proteomics discovery pipeline to a novel set of matched PAH and normal blood, lung and PAEC to identify diagnostic/prognostic IPAH biomarkers. These methods will allow for the first time the potential for specific lung biomarkers of IPAH and possibly PAH in general providing a wealth of new data for future clinical trials and development of new therapeutic targets. The significance is that although there have been advances in therapy and survival with IPAH, the etiology is still largely unknown. In addition we lack the diagnostic/prognostic means to easily and accurately identify patients that are unresponsive to therapy. Therefore biomarkers could fill this void providing the means to decrease therapeutic morbidity, appropriately escalate therapy and serve as measures to benchmark new therapies.